Electret articles—that is, dielectric articles that exhibit at least quasi-permanent electric charge—are known to exhibit good filtration properties. These articles have been fashioned in a variety of constructions, but for air filtration purposes, the articles commonly take the form of a nonwoven polymeric fibrous web. An example of such a product is the Filtrete™ brand furnace filter sold by the 3M Company. Nonwoven polymeric electret filters also have been used in personal respiratory protection devices—see, for example, U.S. Pat. No. 4,536,440 to Berg, U.S. Pat. No. 4,807,619 to Dyrud et al., U.S. Pat. No. 5,307,796 to Kronzer et al., U.S. Pat. No. 5,804,295 to Braun et al., and U.S. Pat. No. 6,216,693 to Rekow et al.
The electric charge enhances the ability of the nonwoven web to capture particles that are suspended in a fluid that passes through the web. The nonwoven web typically contains fibers that comprise dielectric—that is, nonconductive—polymers. A variety of techniques have been developed over the years for producing electret articles.
Early work relating to electrically-charging polymer foils is described by P. W. Chudleigh in Mechanism of Charge Transfer to a Polymer Surface by a Conducting Liquid Contact, 21 APPL. PHYS. LETT., 547-48 (Dec. 1, 1972), and in Charging of Polymer Foils Using Liquid Contacts, 47 J. APPL. PHYS., 4475-83 (October 1976). Chudleigh's method involves charging a polyfluoroethylene polymer foil by applying a voltage to the foil. The voltage is applied through a conducting liquid that contacts the foil surface.
An early-known technique for making a polymeric electret in fibrous form is disclosed in U.S. Pat. No. 4,215,682 to Kubic and Davis. In this method, the fibers are bombarded with electrically-charged particles as they issue from a die orifice. The fibers are created using a “melt-blowing” process, where a stream of gas, which is blown at high velocity next to the die orifice, draws out the extruded polymeric material and cools it into a solidified fiber. The bombarded melt-blown fibers accumulate randomly on a collector to create the fibrous electret web. The patent mentions that filtering efficiency can be improved by a factor of two or more when the melt-blown fibers are electrically-charged in this fashion.
Fibrous electret webs also have been produced by corona charging. U.S. Pat. No. 4,588,537 to Klaase et al., for example, shows a fibrous web that is continuously fed into a corona discharge device while positioned adjacent to one major surface of a substantially-closed dielectric foil. The corona is produced from a high-voltage source that is connected to oppositely-charged thin tungsten wires. Another high-voltage technique for imparting an electrostatic charge to a nonwoven web is described in U.S. Pat. No. 4,592,815 to Nakao. In this charging process, the web is brought into tight contact with a smooth-surfaced ground electrode.
Fibrous electret webs also may be produced from polymer films or foils, as described in U.S. Pat. Re. 30,782, Re. 31,285, and Re. 32,171 to van Turnhout. The polymer films or foils are electrostatically charged before being fibrillated into fibers that are subsequently collected and processed into a nonwoven fibrous filter.
Mechanical approaches too have been used to impart an electric charge to fibers. U.S. Pat. No. 4,798,850 to Brown describes a filter material that contains a mixture of two different crimped synthetic polymer fibers that have been carded into a fleece and then needled to form a felt. The patent describes mixing the fibers well so that they become electrically-charged during the carding. The process disclosed in Brown is commonly referred to as “tribocharging”.
Tribocharging also can occur when high-velocity uncharged jets of gases or liquids are passed over the surface of a dielectric film. In U.S. Pat. No. 5,280,406, Coufal et al. disclose that when jets of an uncharged fluid strike the surface of the dielectric film, the surface becomes charged.
A more recent development uses water to impart electric charge to a nonwoven fibrous web (see U.S. Pat. No. 5,496,507 to Angadjivand et al.). Pressurized jets of water or a stream of water droplets are impinged onto a nonwoven web that contains nonconductive microfibers to create the electric charge. Subjecting the web to an air corona discharge treatment before the hydrocharging operation can further enhance charging. The resulting charge provides filtration-enhancing properties. See also U.S. Pat. Nos. 6,824,718 and 6,406,657 to Eitzman et al., and U.S. Pat. Nos. 6,783,574, 6,375,886, and 6,119,691 to Angadjivand et al., and U.S. Pat. No. 6,743,464 to Insley et al., for other methods of using water to produce electret articles. Nonaqueous polar liquids too have been used in making fibrous electret articles—see U.S. Pat. No. 6,454,986 to Eitzman et al.
Adding certain additives to the web has improved electret performance. One method for improving resistance to oily-mist aerosols, includes adding a melt processable fluorochemical additive such as a fluorochemical oxazolidinone, a fluorochemical piperazine, or a perfluorinated alkane to the polymer during the creation of the polymeric fibrous article—see, for example, U.S. Pat. Nos. 5,025,052 and 5,099,026 to Crater et al. and U.S. Pat. Nos. 5,411,576 and 5,472,481 to Jones et al. The fluorochemicals are melt processable, that is they suffer substantially no degradation under the melt processing conditions that are used to form the fibers in the electret web—see also U.S. Pat. No. 5,908,598 to Rousseau et al. In addition to a melt-processing method, fluorinated electrets also have been made by placing a polymeric article in an atmosphere that contains a fluorine-containing species and an inert gas and then applying an electrical discharge to modify the surface chemistry of the polymeric article. The electrical discharge may be in the form of a plasma such as an AC corona discharge. The plasma fluorination process causes fluorine atoms to become present on the surface of the polymeric article. The fluorinated polymeric article may be electrically charged using, for example, the hydrocharging techniques mentioned above. The plasma fluorination process is described in a number of U.S. Pat. Nos. 6,397,458, 6,398,847, 6,409,806, 6,432,175, 6,562,112, 6,660,210, and 6,808,551 to Jones/Lyons et al. Electret articles that have a high fluorosaturation ratio are described in U.S. Pat. No. 7,244,291 to Spartz et al., and electret articles that have a low fluorosaturation ratio, in conjunction with heteroatoms, is described in U.S. Pat. No. 7,244,292 to Kirk et al. Other publications that disclose fluorination techniques include: U.S. Pat. Nos. 6,419,871, 6,238,466, 6,214,094, 6,213,122, 5,908,598, 4,557,945, 4,508,781, and 4,264,750; U.S. Publications US 2003/0134515 A1 and US 2002/0174869 A1; and International Publication WO 01/07144.
Filter webs also have been produced without deliberately post-charging or “electreting” the fibers or the fiber webs—see U.S. Pat. No. 5,780,153 to Chou et al. The fibers are made from a copolymer that comprises: a copolymer of ethylene, 5 to 25 weight percent of (meth)acrylic acid, and optionally, though less preferably, up to 40 weight percent of an alkyl (meth)acrylate whose alkyl groups have from 1 to 8 carbon atoms. Five to 70% of the acid groups are neutralized with a metal ion, particularly an ion of zinc, sodium, lithium, or magnesium, or a mixture of these. The copolymer has a melt index of 5 to 1000 grams (g) per 10 minutes. The remainder may be a polyolefin such as polypropylene or polyethylene. The fibers may be produced through a melt-blowing process and may be cooled quickly with water to prevent excess bonding. The patent discloses that the fibers have high static retention of any existing or deliberate, specifically induced, static charge.
Other electrets that contain additives are described in U.S. Pat. No. 5,057,710 to Nishiura. The polypropylene electrets disclosed in Nishiura contain at least one stabilizer selected from hindered amines, nitrogen-containing hindered phenols, and metal-containing hindered phenols. The patent mentions that an electret that contains these additives can offer high heat-stability. The electret treatment was carried out by placing the nonwoven fabric sheet between a needle-like electrode and an earth electrode. U.S. Pat. Nos. 4,652,282 and 4,789,504 to Ohmori et al. describe incorporating a fatty acid metal salt in an insulating polymer to maintain high dust-removing performance over a long period of time. Japanese Patent Kokoku JP60-947 describes electrets that comprise poly 4-methyl-1-pentene and at least one compound selected from (a) a compound containing a phenol hydroxy group, (b) a higher aliphatic carboxylic acid and its metal salts, (c) a thiocarboxylate compound, (d) a phosphorous compound, and (e) an ester compound. The patent indicates that the electrets have long-term storage stability.
Other Japanese publications that disclose methods for producing electret articles include 2002-115178, 2002-115177A, 2003-013359, 2004-66026, 2004-66027, and 2004-195357.
For a brief survey of the ion chemistry at the interfaces and of how ions may be transferred, facilitated by adsorbed water to produce electrets, see McCarty and Whitesides, Electrostatic Charging Due to Separation of Ions at Interfaces: Contact Electrification of Ion Electrets, 47 ANGW. CHEM. INT. 2-22 (2008); see also McCarty et al., Ionic Electrets: Electrostatic Charging of Surfaces by Transferring Mobile Ions Upon Contact, 129 J. AM. CHEM. SOC. 4075-88 (2007), and Kudin et al., Why Are Water—Hydrophobic Intersurfaces Interfaces Charged?, J. AM. CHEM. SOC. (received Sep. 17, 2007).